Carbonaceous chars such as activated carbons, especially those produced at high temperatures (i.e., above 700.degree. C.), often possess catalytic properties which may interfere with their use in certain applications. The origins of this catalytic activity can usually be attributed to two principal factors: (1) the inorganic non-carbon ash constituents of the char, e.g. iron, potassium, and calcium, and (2) the inherent catalytic properties of the carbon itself. If the char is used for the physical adsorption, removal, and/or recovery of substances from fluid streams, the presence of catalytic reactivity in the char may lead to an unwanted chemical conversion of the adsorbates into materials that contaminate the final process stream or interfere with the physical adsorption process. Moreover, the reactions which lead to catalytic chemical conversion are often highly exothermic, increasing the likelihood of ignition of the char itself.
Examples of processes in which the catalytic reactivity of the char is a liability include the purification of reactive or oxidizable products such as hydrogen peroxide and organic glycols, and the adsorption, recovery, and re-use of reactive or oxidizable solvents such as acetone and methyl ethyl ketone. In these types of applications, the primary adsorbate or component of the stream is itself a material which can become reactive if catalyzed by the carbon under certain conditions. In other applications it may be desirable to preserve a reactive constituent, e.g. hydrogen peroxide, such that it is available for reaction with species other than the carbon itself Less obvious is the need for a non-catalytic, adsorptive carbon for applications in which the potentially reactive component is an interference, constituting only a small part of the overall adsorbate loading. For example, in streams containing small concentrations of an oxidizable material such as hydrogen sulfide and much higher concentrations of an adsorbable, recoverable organic, it is possible that the oxidation of the hydrogen sulfide, which is only weakly physically adsorbed per se, can cause a build-up over time of highly adsorbable and polar reaction products, such as sulfuric acid, which can greatly interfere with the adsorption of the organic components of the stream. If the inherently reactive carbon catalytic sites can be deactivated, maximum use can be made of the physical adsorption properties of the carbon for the removal and recovery of the organic stream components.
Prior art methods to reduce the catalytic reactivity of carbonacous chars have been directed almost exclusively on the removal or deactivation of catalytically-active ash constituents which may be present in the char. For example, it is known to remove acid-soluble ash constituents by acid-washing of the char. After treatment, the char is invariably rinsed with water and/or an aqueous solution of a base to remove and/or neutralize the acid. Hydrofluoric acid solution, an extremely hazardous material, has been found to be particularly effective for removing such ash components. Ash reductions exceeding an order of magnitude are usually achievable by these methods. Other ash removal processes treat the char first with caustic solution to remove caustic-soluble components, followed by the aforementioned acid extraction treatment to remove the remaining acid-soluble components. In other methods, the ash components are treated with agents such as silanes to reduce their catalytic reactivity without removing them from the carbon surface. In all of these prior art methods, however, little regard has been given to the inherent reactivity of the carbon itself, which alone may be sufficient to interfere with the intended function of the char.
Where the reactivity of the carbon has been considered, no methods have been identified or reported which can deactivate the carbon effectively and irreversibly. For example, it has been observed that oxygen can chemisorb to the carbon surface and cause the carbon to become less reactive. However, after use, and upon high-temperature thermal treatment of the char to remove other adsorbed materials, it is found that oxygen is also lost from the carbon. Upon removal of the oxygen, the inherent catalytic reactivity of the carbon once again becomes manifest, requiring additional post-treatment of the thermally-treated char to deactivate the carbon prior to re-use. Therefore, the oxygen appears to have merely masked, and not destroyed, the catalytic activity of the carbon.
Accordingly, it is the object of the present invention to provide a process which reduces the inherent carbon catalytic activity of carbonaceous chars for use in those applications wherein the inherent catalytic activity of the carbon is a concern. It is further the object of the present invention to provide a process in which the required deactivation of the carbon can be accomplished in a convenient and economical manner, and be readily integrated into contemporary methods for the production of carbonaceous chars, in particular the production of activated carbons. Additionally, it is the object of the present invention to provide a method for the deactivation of the carbon in such a way that the deactivation is largely irreversible upon high-temperature thermal treatment. It is also an object of the invention to provide a carbonaceous char having substantially reduced inherent catalytic activity.